Process for alumina recovery

Abstract

Aqueous suspensions of red mud are effectively removed from the Bayer process for making alumina by the addition to at least the first stage of the caustic recovery circuit of a flocculant selected from the group consisting of starch, homopolymers of acrylic acid or acrylates, copolymers of acrylic acid or acrylates containing at least 80 molar percent acrylic acid or acrylate monomers and combinations thereof and subsequent addition to later, more dilute stages in the caustic recovery circuit of a copolymer containing from about 35 to 75 molar percent of acrylic acid or acrylate and from about 65 to 25 molar percent of ethylenically unsaturated polymerizable monomers.

What is claimed is: 
     
       1. A process for flocculating red muds from the Bayer alumina recovery circuit which comprises adding to at least a first stage of the caustic recovery circuit an effective amount of a flocculant selected from the group consisting of starch, homopolymers of acrylic acid or acrylates, copolymers of acrylic acid or acrylates containing at least 80 molar percent acrylic acid or acrylate monomers and combinations thereof and thereafter adding to some subsequent stage or stages of the caustic recovery circuit an effective amount of a copolymer containing from about 35 to 75 molar percent of acrylic acid or acrylate monomers and from about 65 to 25 molar percent of acrylamide monomers. 
     
     
       2. The process of claim 1 wherein the flocculant added to at least a first stage of the caustic recovery circuit is a copolymer of acrylic acid or acrylate containing at least 90 molar percent acrylic acid or acrylate monomers and no more than 10 molar percent acrylamide monomers. 
     
     
       3. The process of claim 1 wherein the flocculant added to some subsequent stage or stages of the caustic recovery circuit is a copolymer containing 50 to 70 molar percent of acrylic acid or acrylate monomers and from 50 to 30 molar percent acrylamide monomers. 
     
     
       4. The process of claim 1 wherein the flocculant added to at least a first stage of the caustic recovery circuit is a copolymer of acrylamide acid or acrylate containing at least 90 molar percent acrylic acid or acrylate monomers and no more than 10 molar percent acrylamide monomers and wherein the flocculant added to some subsequent stage or stages of the caustic recovery circuit is a copolymer containing 50 to 70 molar percent of acrylic acid or acrylate monomers and from 50 to 30 molar percent acrylamide monomers. 
     
     
       5. The process of claim 1 wherein the effective amount of the first added flocculant is between 0.01 and 2.0 percent by pounds of flocculant per ton of dry mud residue and the effective amount of the subsequently added flocculant is between 0.01 and 1.0 pound of flocculant per ton of dry mud residue. 
     
     
       6. The process of claim 1 wherein the subsequently added flocculant is added to a stage or stages wherein the NaOH plus Na 2  CO 3  concentration of the solution is less than about 100 grams per liter. 
     
     
       7. The process of claim 1 wherein the subsequently added flocculant is added to a fourth or latter stages of the caustic recovery circuit. 
     
     
       8. The process of claim 1 wherein the subsequently added flocculant is a copolymer containing 70 molar percent sodium acrylate and 30 molar percent acrylamide with a molecular weight in the range of 5-10 million.